f internal parasite
infection includes treatment expense,
reduced animal weight gains, and even
animal death.
These parasites are dif cult to manage
because on some farms they have devel-
oped resistance to all available commer-
cial dewormers. (Zajac, Gipson, 2000)
Resistance to dewormers is now seen
worldwide (Kaplan, 2004b). Producers
can no longer rely on drugs alone to con-
trol internal parasites. Rather, an inte-
grated approach that relies on sustain-
able methods to manage internal parasites
should be employed.
Internal parasite management, especially of Haemonchus contortus (barberpole worm, stomach worm),
is a primary concern for the majority of sheep and goat producers. These parasites have become more
dif cult to manage because of developed resistance to nearly all available dewormers. This publication
discusses new techniques to manage parasites and to prolong the ef cacy of dewormers. New manage-
ment tools that remain under investigation are also discussed. A list of resources follows the narrative.
A Publication of ATTRA - National Sustainable Agriculture Information Service 1-800-346-9140 www.attra.ncat.org
ATTRANational Sustainable
Agriculture Information Service
is managed by the National Cen-
ter for Appropriate Technology
(NCAT) and is funded under a
grant from the United States
Department of Agricultures
Rural Business-Cooperative Ser-
vice. Visit the NCAT Web site
(www.ncat.org/agri.
html) for more informa-
tion on our sustainable
agriculture projects.
ATTRA
Introduction ..................... 1
Parasite Primer ................ 2
Parasitism .......................... 2
Resistance to
Dewormers ....................... 3
Pasture Management ... 4
New Techniques ............. 5
Other Techniques ........... 6
Conclusion ........................ 7
Resources .......................... 7
References ........................ 8
Contents
By Margo Hale
NCAT Agriculture
Specialist
© NCAT 2006
Managing Internal Parasites
in Sheep and Goats
Owners of this Katahdin ewe and her lambs are able to manage internal parasites using sustainable techniques.
NCAT photo by Margo Hale. Page 2
ATTRA
Managing Internal Parasites in Sheep and Goats
Parasite Primer
Internal parasites (worms) exist by feeding
off of their host. Some types do this directly,
by attaching to the wall of the digestive sys-
tem and feeding on the hosts blood. These
types of parasites cause anemia in the host,
as well as other symptoms. Haemonchus
contortus (barberpole worm) is one exam-
ple of this type. Others live off the nutrients
eaten by the host; these cause weight loss
but not anemia.
Mature parasites breed inside the host
and lay eggs, which pass through the
host and are shed in the feces. After the
eggs pass out of the host, they hatch into
larvae. Warm, humid conditions encour-
age hatching. The larvae need moisture to
develop and move. They migrate out of the
feces and up blades of grass (usually 1 to
2 inches). When an animal
(sheep or goat) grazes, they
may take in parasite larvae
along with the grass blade.
An animal can also pick up
parasite larvae by eating
from a feed trough that is
contaminated by manure.
Parasite numbers increase
over time when conditions
are favorable (warm, wet).
Internal parasites get out
of control and cause dam-
age when their numbers
grow beyond what the animal can tolerate.
In order to manage internal parasites, it is
important to understand the parasite cycle
and factors that encourage their production.
Parasitism
Animals raised in con nement or on pas-
ture-based systems will almost certainly be
exposed to internal parasites at some point
in their lives. Dry environments, such as
arid rangelands, will pose less of a threat
for parasite infections. Warm, humid cli-
mates are ideal for worms, and therefore
animals will have more problems with inter-
nal parasites in these climates.
Sheep and goats should be managed so that
parasitism is not evident. Sheep and goats
will always host some level of parasite bur-
den. Certain signs of parasitism are seen
when the parasite load becomes excessive
or when the animals immunity can no lon-
ger overcome the adverse effects of the par-
asitism. (Scarfe, 1993) Young animals and
those with weakened immune systems due
to other diseases are most affected by inter-
nal parasitism. A combination of treatment
and management is necessary to control
parasitism so that it will not cause economic
loss to the producer. (Scarfe, 1993)
While it is ideal to manage animals so there
are no visible effects of parasitism, some
will nonetheless succumb to the burden of
internal parasites. Learn to recognize the
signs of internal parasite infections and
offer early treatment.
Due to lowered immunity, young stock and pregnant
or lactating animals are more likely to be a ected by
internal parasites. Photo by Linda Co ey.
Most animals in a ock are not visibly a ected by parasites and do not need to be
treated with dewormers. Photo by Linda Co ey.
Internal parasite numbers: Increase with number of host animals Increase during warm, humid
weather Increase when pastures are grazed
too short Decrease during hot, dry weather Decrease if a non-host animal (cattle
or horses) graze the same pasture Decrease with pasture rest time, as
the larvae naturally die o Page 3
ATTRA
www.attra.ncat.org
Resistance to Dewormers
Producers were once instructed to deworm
all of their animals every three to six
months. Many producers dewormed even
more often, as often as every four weeks in
humid climates. It is now known that this
practice is not sustainable.
Drug resistance is the ability of worms in a
population to survive drug treatments that
are generally effective against the same spe-
cies and stage of infection at the same dose
rate. (Kaplan, 2004b) Over-use of deworm-
ers has led to resistance, and available
dewormers are now ineffective. In an arti-
cle from 1993, David Scarfe predicted the
development of drug resistance.
Suppressive deworming is probably the
most effective means of keeping parasite
numbers lowered for a period of time. How-
ever, this method will also eventually lead
to resistance to the anthelminthics(s) used
much more rapidly than if other strategies
of control are utilized. One point to con-
sider here is alternating the use of differ-
ent drugs.
It is considered by this author, and sev-
eral expert parasitologists, that rapid rota-
tion of different drugs is ill-advised as this
will lead to resistance of multiple drugs
something that the small ruminant indus-
tries certainly do not need. (Scarfe, 1993)
Scarfe recognized the unsustainable prac-
tices that were being used long before para-
sites were resistant to dewormers in the U.S.
Some farms still have dewormers that
continue to work, while others have no
effective dewormers. This is a problem
because no new dewormersc for sheep and
goats are currently under development.
(Kaplan, 2004b)
Development of Resistance
to Dewormers
Internal parasites, especially H. contortus,
have developed drug resistance. Drug treat-
ment gets rid of the worms that are suscep-
tible to that particular drug; resistant para-
sites survive and pass on resistant genes.
Signs of Parasitism

Loss of condition
Rough hair coat
Scours, diarrhea
Bottle jaw
Pale mucous membranes (eyelids, gums),
indicating anemia
Death
Bottle jaw is a sign of parasitism.
Photo courtesy of Jean-Marie Luginbuhl.
Loss of condition and rough hair coat indicate parasitism.
Photo courtesy of Jean-Marie Luginbuhl. Page 4
ATTRA
Managing Internal Parasites in Sheep and Goats
Overview of Available Dewormers for Sheep and Goats
Several types of dewormers are available for use in sheep and goats. Many are not approved for use in sheep and goats, how-
ever, so work with a veterinarian to ensure proper o -label use. The di erent classes of dewormers have di erent modes to kill
worms. The level of resistance depends on the class of dewormer and how often the drug was used on a particular farm.
Drug Class
Common Names/ Brands
E ectiveness
Benzimidazoles
Albendazole (Valbazen®),
Fenbendazole (Safeguard®)
High prevalence of resistance
Avermectin/
Milbemycins
Ivermectin (Ivomec®)
Moxidectin
(Cydectin®)
Ivermectin least e ective of all
available drugs
Moxidectinresistance becoming
common where used frequently
Imidazothiazoles/
Tetrahydropyrimidine
Levamisole (Tramisol®),
Pyrantel (Strongid®),
Morantel (Rumatel®)
Low to moderate prevalence of
resistance
Worms that are not treated are called
refugia. The concept of refugia has been
largely overlooked in the past. Having some
worms in refugia (not treated) insures that
a level of genes remain sensitive to deworm-
ers. (Kaplan, n.d.) A surviving population
of untreated worms dilutes the frequency
of resistant genes. Consequently, when a
dewormer is required, it will be effective
because the worms will be susceptible to
treatment. (Kaplan, n.d.)
When fewer numbers of animals receive
treatment, the refugia population remains
large. The more refugia, the better. Sustain-
able techniques, such as FAMACHA
©
, ght
drug resistance by increasing refugia.
In contrast, several practices accelerate
drug resistance. They include frequent
deworming (more than three times a year),
underdosing (often caused by miscalcula-
tion of body weight), treating and moving
to clean pasture, and treating all animals,
regardless of need. These practices lead
to resistance because they decrease the
number of worms susc